The fact that you need other metrics does not substract from OPs original point. It's still good and much better overall to handle a set of comprehensive metrics at infra level than to orchestrate every app.
About the coarsness, i think it's not really true. Proxies are freaking powerful and they do a lot of stuff at l7, too much in fact (look at envoy, jesus christ). That's one of the reasons why despite the insane complexity of service meshes, they are paramount for observability.
The fact that you need other metrics does not substract from OPs original point. It's still good and much better overall to handle a set of comprehensive metrics at infra level than to orchestrate every app.
About the coarsness, i think it's not really true. Proxies are freaking powerful and they do a lot of stuff at l7, too much in fact (look at envoy, jesus christ). That's one of the reasons why despite the insane complexity of service meshes, they are paramount for observability.